bridge and World War II delayed any further work on a structure beyond tests of the lake bottom and the construction of the causeway on the St. Ignace side; the first bridge authority was abolished in 1947. The current agency was created on June 6, 1950. The MBA was authorized in 1952 to sell bonds to finance construction, which were sold on December 17, 1953, to finance the $99.8 million (equivalent to $ in ) cost of the bridge. The structure was designed by David B. Steinman and built by Merritt-Chapman & Scott for the substructure and the American Bridge Company division of U.S. Steel Corporation for the superstructure. Construction started in 1954 and the Mackinac Bridge opened to traffic on November 1, 1957. Final work on the bridge was completed in September 1958. Overall, the structure has a central suspension span flanked by two side spans. With the two backstay spans, the Mackinac Bridge is long between cable anchorages, the longest in the world at the time it opened. The total length of the structure is with two towers and of clearance for passing ships under the main span. In 2000, the bridge was named "Michigan's No. 1 Civil Engineering Project of the 20th Century" by the Michigan Section of the American Society of Civil Engineers (ASCE), and the structure was named a National Historic Civil Engineering Landmark in 2010 by the national ASCE. The northernmost of the three monumental bridges along I-75 is the International Bridge, linking the twin cities of Sault Ste. Marie in Michigan and Ontario. The governments on each side of the international border formed a bridge authority to build a highway bridge in 1935. Construction started on the structure September 16, 1960. The International Bridge is nearly long, encompassing spans over the American and Canadian navigation channels for the Soo Locks and the St. Marys River. The American approach is , and the Canadian approach is . The center span over the river is , flanked by and spans over the American and Canadian shipping channels, respectively. The bridge was designed by the same firm that handled the Mackinac Bridge for a cost of $20 million (equivalent to $ in ). It opened to traffic on October 31, 1962. Exit list Related trunklines There are four auxiliary Interstate Highways for I-75 in Michigan. I-275 begins as a loop from northern Monroe County and continues